A strain sensitive electrical device mounted on a substrate will undergo deformation with the substrate, and the strain measured by the device can thereby provide a measure of the deformation in the substrate itself. Such strain sensitive devices can be incorporated in pressure sensors--for example, with the strain sensitive device directly mounted on a pressure sensitive diaphragm or membrane--, and in other dynamic devices such as accelerometers, as well as being used for the direct measurement of static strain in a structural member.
A common manner of measuring the strain in a mechanical member--and therefore its relative deformation--is to form a piezoresistive conductor on the substrate member which is then biased at a desired voltage or current level. A deformation of the substrate causes a strain field in the piezoresistive conductor which changes its resistance, resulting in a variation in the voltage across or the current through the conductor which can be related to the strain imposed upon it. Various common semiconductors are among the materials that exhibit piezoresistivity. A uniformly doped piezoresistive semiconductor, at moderate biasing potentials, will behave as a linear resistor which obeys Ohm's law. The imposition of strain on the semiconductive resistor will change the value of its resistance with a resulting measurable change in voltage across the resistor (if constant current is applied) or change in current through the resistor (if constant voltage is applied). The changes in the resistance of such piezoresistive semiconductors are generally considered to be due to changes in the geometry of the resistor (for example, for a simple rectangle bar resistor, a change in the ratio of area to length will change its resistance), and changes in the active carrier concentration and in the low field carrier mobility.
An important measure of the sensitivity of a strain measuring device is its gauge factor, which, for small signals, can be defined as the ratio of the change in resistance due to strain over the normal (unstrained) resistance divided by the magnitude of the strain that caused the change in resistance. For a piezoresistive semiconductor operated in its linear range, the gauge factor is not a function of the applied voltage or current because the resistance of the semiconductor does not change with the applied voltage or current. A large gauge factor is preferred since the lower the gauge factor the higher the amplification required to provide a usable output signal to processing circuitry, which increases the noise content of the output signal as well as increasing the complexity and cost of the signal conditioning circuitry.